The present disclosure relates to an image forming apparatus and an image forming method for forming an electrophotographic image.
The electrophotographic image forming apparatus such as the printer or the copying machine forms an image by performing a series of steps including the charging step, the exposure step, the developing step, the transferring step, and the fixing step. The charging step is for uniformly electrifying a photosensitive drum. The exposing is for exposing an electrified photosensitive drum and forming an electrostatic latent image thereon. The developing is for adhering toners to the electrostatic latent image and forms a visible image. The transferring is for transferring the visible image to a paper. The fixing is for fusing and fixing the visible image transferred to the paper.
In this kind of image forming apparatus, the electrostatic latent image is formed on an image carrying surface of the photosensitive drum, and a developing roller for developing the electrostatic latent image is disposed so as to face the image carrying surface. A surface of the developing roller carries the toner that is the developer. At developing the electrostatic latent image, the development field is generated between the developer carrying surface of the developing roller and the image carrying surface of the photosensitive drum, and the toner is adhered to the electrostatic latent image by the action of the development field.
In order not to change a distance between the image carrying surface and the developer carrying surface facing each other because of the rotation of the photosensitive drum and the rotation of the developing roller, a rotatory shaft of the photosensitive drum is disposed so as to be parallel to a rotatory shaft of the developing roller. In the image forming apparatus assembled in fact, however, the rotatory shaft of the photosensitive drum and the rotatory shaft of the developing roller are disposed within a specific tolerance. That is to say, strictly speaking, there is a possibility that following states appear in the main scanning direction of the photosensitive drum after the assembly; e.g. the rotatory shaft of the photosensitive drum is close to the rotatory shaft of the developing roller at one end, while the rotatory shaft of the photosensitive drum is apart from the rotatory shaft of the developing roller at the other end.
When a constant potential difference is applied between the image carrying surface and the developer carrying surface in order to generate the development field, the intensity of the development field becomes small at a position where the distance between the image carrying surface and the developer carrying surface is large, on that the density of the visible image becomes thin. In addition, the intensity of the development field becomes large at a position where the distance between the image carrying surface and the developer carrying surface is small, so that the density of the visible image becomes dense. That is to say, in the main scanning direction, if the distance between the rotatory shaft the photosensitive drum and the rotatory shaft of the developing roller varies according to a position, it occurs that there is a difference in the amount of toner to be adhered according to the position in the main scanning direction. The variation of the toner adhering amount appears as the density unevenness of the visible image, which deteriorates the picture quality. In order to prevent the density unevenness, the above-mentioned tolerance is controlled to be a very small value.
There is an image forming apparatus for controlling the above-mentioned tolerance, wherein, after the distribution of the distance between the photosensitive drum and the developing roller in the main scanning direction has been recorded at assembling the process cartridge including drum periphery members such as the photosensitive drum, the developing roller, and so on, the image forming is executed according to the recorded data. In such image forming apparatus, the intensity of the light beam for irradiating the image carrying surface at the developing can be controlled according to the recorded data, that is, it is possible to reduce the intensity of the light beam at the position on which the distance between the photosensitive drum and the developing roller is small, and increase the intensity at a position on which the distance between the photosensitive drum and the developing roller is large.
On the assumption that the developer carrying surface is exactly parallel to the rotatory shaft of the developing roller and there is no variation of the distance between the photosensitive drum and the developing roller after the assembling, the above-mentioned art can prevent the density unevenness.
However, in the developing roller manufactured in real, the developer carrying surface is not exactly parallel to the rotatory shaft of the developing roller and a curve of approximately dozens of μm is generated on the developer carrying surface. When the developing roller rotates, the shaft deviation, the developer carrying surface approaches and separates from the image carrying surface, occurs because of the curve. As a result, the distance between the image carrying surface and the developer carrying surface varies along with the rotation of the photosensitive drum and the rotation of the developing roller.
In addition, the rotation speed of the photosensitive drum is not always identical with the rotation speed of the developing roller, and the circumference of the photosensitive drum is not constant multiples of the circumference of the developing roller. That is to say, even if the position is the same on the photosensitive drum, the distance between the image carrying surface and the developer carrying surface varies whenever the photosensitive drum rotates. When the above-mentioned art is applied to the image forming apparatus wherein the distance between the image carrying surface and the developer carrying surface varies intermittently, it happen to increase the intensity of the light beam regardless of the small distance between the image carrying surface and the developer carrying surface, and reduce the intensity of the light beam regardless of the large distance between the image carrying surface and the developer carrying surface. Therefore, there is a possibility to deteriorate the density unevenness moreover by applying the above-mentioned art to the image forming apparatus.
In order to reduce the density unevenness caused by the shaft deviation, there is an image forming apparatus configured so as to detect the variation of the distance between the image carrying surface and the developer carrying surface as a capacitance, and change a developing AC voltage and a developing DC voltage applied for generating the development field according to the capacitance.
In such image forming apparatus, since the developing AC voltage and the developing DC voltage varies according to the capacitance between the image carrying surface and the developer carrying surface, it is possible to change the intensity of the development field according to the variation of the distance between the image carrying surface and the developer carrying surface. Therefore, as compared with the configuration wherein the developing AC voltage and the developing DC voltage do not vary, it is possible to reduce the density unevenness of the visible image caused by the variation of the distance between the image carrying surface and the developer carrying surface.
It is easy for human sight to recognize the density unevenness at the high printing density rather than the low printing density. For instance, when the variation amounts of the distance between the image carrying surface and the developer carrying surface are the same, the human visual sense recognizes that the density unevenness of the visual image in the 20% density is larger than the density unevenness of the visual image in the solid black of the 100% density. This image forming apparatus is configured so as to reduce an absolute value of a contrast potential by 10% when a detection voltage of capacitance between the image carrying surface and the developer carrying surface raises by 10%, and increase the absolute value of the contrast potential by 10% when the detection voltage of capacitance between the image carrying surface and the developer carrying surface falls by 10%. Therefore, in the image forming apparatus, the visual effect for the human is not taken into consideration at all.
As described above, the rotation speed of the photosensitive drum is not identical with the rotation speed of the developing roller, and the distance between the image carrying surface and the developer carrying surface varies even at the same position on the image carrying surface whenever the photosensitive drum rotates. Since the image forming apparatus is configured that the developing AC voltage and the developing DC voltage are changed according to a mean value of the capacitances between the image carrying surface and the developer carrying surface, the capacitances obtained at plural points dividing the circumference of the photosensitive drum into equal parts, the apparatus cannot deal with those variations.